Magnesium alloys



United States Patent M 3,063,834 MAGNESIUM ALLOYS Geolfrey CharlesEdward Olds, Leicester, England, as-

signor to Associated Electrical Industries (Rugby) Lim-' ited, London,England, a British company No Drawing. Filed Oct. 20, 1958, Ser. No.768,062 Claims priority, application Great Britain Oct. 25, 1957 2Claims. (Cl. 75-468) This invention relates to magnesium alloyssuitable, inter alia, for use as canning materials for the fuel elementof nuclear reactors.

The important mechanical properties required in a canning material foruranium fuel in a thermal nuclear reactor are high creep ductilitybetween 200 C. and 500 C.; small grain size and high stability of grainsize up to 500 C.; freedom from excessive intercrystalline cavitation;high creep strength at low stresses at the highest operatingtemperatures and good weldability.

Alloys used for cans in natural uranium reactors must also possess verylow neutron absorption cross-sections and for this reason magnesium isthe most important metal to form the main constituent of any alloy used.Any alloying elements used must be such that the neutron absorptioncross-section of the resulting alloy is adequately small. Magnesiumalloys which have been considered up to now have been those in which themain alloying constituent is approximately 1% aluminium, magnesiumalloys containing 0.5% zirconium, and those containing 0.5% zirconiumwith 0.5 %l% zinc.

The two alloys of magnesium with 0.5% zirconium, and magnesium with 0.5zirconium and 0.5 %-1% zinc have many properties which make themsuitable for reactor cans with the exception'that their creep strengthat high temperatures of the order 500 C. is insufficiently high.

We have found that a small quantity of manganese alloyed in magnesiumwith zirconium or with zirconium and zinc produces an alloy with highcreep resistance, yet still possessing the other desirable propertieswhich are known to exist in binary magnesium-zirconium or ternarymagnesium-zirconiumzinc alloys. These improved alloys a-re essentiallysingle-phase alloys, which at high temperatures do not exhibitprecipitation hardening which is usually associated with creepresistance.

An alloy according to the invention consists of zirconium, magnesium andmanganese, and optionally zinc, the proportions, by weight, in the alloylbeing 0.3-1.0% Zr, 0.050.5% Mn, 02.0% Zn and the remainder magnesium.

It is known that the useful liquid and solid solubility of Zr in Mg toform a single phase alloy is about 0.7%, above which percentage azirconium precipitate is present in the final alloy. Also manganese hasa limited liquid and solid solubility in magnesium, such that if theamount present in a binary alloy exceeds about 0.5% an undesirableprecipitate is present in the alloy, which may lead to embrittlement. IfZr and Mn are both present in an Mg alloy (with or without the optionalzinc content), each of the two elements limits the solubility of theother, i.e. a small Mn addition of a few tenths of 1% reduces thesolubility of Zr to less than 0.7%. The presence of zinc in thequantities mentioned makes little difference to the aforementionedsolubility and precipitation problems. The incorporation of Mn in themanufacture of Mg.Zr alloys (with or without zinc) thereforenecessitates the reduction of the Zr content to less than 0.7%, both ongrounds of obtaining the alloy elements in solution in the melt, and inproducing a final alloy of single phase, which for the latter reasonshould also be free from serious em- 3,063,834 Patented Nov. 13, 1962brittlement; TheZr andMn contents lof any Mg.Zr.Mn(Zn) alloys are,therefore, interdependent.

Furthermore, the preferred maximum Mn content for a canning material ongrounds of neutron absorption economy is about 0.2%.

The preferred compositions according to this invention have been foundto be within the following ranges: 0.3%-0.8% Zr, 0.05%0.3% Mn, remainderMg.

In more detail, the following examples are given of preferredcompositions which were found to be essentially single phase alloys:

Alloy (1) 0.5% Zr, 0.12% Alloy (2) 0.5% Zr, 0.12% Alloy (3) 0.3% Zr,0.20% Mn, remainder Mg Alloy (4) 0.3% Zr, 0.20% Mn, 0.5% Zn, remainderMg A comparison of the alloys given in Examples ('3) and (4), with thecorresponding alloys given in Examples (1) and (2), illustrate themanner in which the proportion of Zirconium is reduced when theproportion of manganese is increased in order to allow for the mutualprecipitation eflFects of these constituents.

The presence of zinc in the alloys improves the resistance to graingrowth at high temperature, which if such growth is high leads toreduced ductility. The improve ment in creep resistance of the alloyscontaining manganese over their respective parent alloys of Mg-Zr(Zn) isshown by tensile test creep results as follows:

Mn, remainder Mg Mn, 0.5 Zn, remainder Mg Alloy Parent Mg. Zr. Zn.

5 999 oaeeulovmace The retention of adequately small grain size andresistance to serious grain growth at the upper temperature (500 C.),and the retention of adequate low temperature ductility (200 C.), in allthe new alloys, both despite the reduced Zr content, is shown in thefollowing tables. Also shown is the efiect of Zn in tending to promoteeven greater grain size stability.

Average Grain Diameter in Mierons after Annealing 500 hours at 500 0.

Alloy Percent Tensile Elongation at Fracture when Percent TensileElongation at Alloy Fracture when 3 4 Whatl elaim is: References Citedin the file of this patent l. An alloy consisting essentially, byWeight, of zirconi- UNITED STATES PATENTS um 0.5%, manganese 0.12%, andthe remainder magnesi- 2,371,531 Mel Donald Mar. 13, 1945 2. An alloyconsisting essentially of, by weight, zir- 5 2,788,272 Whltehead at 1957conium 0.3%,manganese 0.2%, and the remainder mag- OTHER REFERENCESnesmm' The Influence of Zirconium Upon the solidification of MagnesiumAlloys and Some of the Properties of Cast 10 Magnesium Alloys ContainingZirconium," by Franz Sauerwald. Zeitschrift fiir Metallkunde, vol. 40,1949, pages 44 and 45.

1. AN ALLOY CONSISTING ESSENTIALLY, BY WEIGHT, OF ZIRCONIUM 0.5%,MANGANESE 0.12%, AND THE REMAINDER MAGNESI-UM..